1. Field of the Invention
The present invention relates to beam generation and scheduling of users in a communication system.
2. Description of Related Art
Opportunistic scheduling (OS) has been proposed as a scheduling technique to improve the downlink throughput of high speed packet systems. OS (as employed in a base station scheduler or scheduling algorithm, for example) exploits the fact that the propagation channels between the base station and the MSs served by the base station fade independently. This independent fading gives rise to what is referred to as multi-user diversity (MUD). Multi-user diversity is a form of diversity that schedules transmissions to users, when their time varying channels approach their top capacity, thus increasing the throughput of the system.
OS exploits the fact that many data services are delay tolerant. However, real time services such as voice, and in some cases video, can accept only a limited amount of packet delay, thus a scheduler should also consider packet delay statistics. Additionally, the issue of fairness should be addressed. Accordingly, a scheduler or scheduling algorithm implementing OS should weigh desired system throughput with fairness and delay statistic considerations.
Several schedulers or scheduling algorithms have been developed in an effort to provide reasonable tradeoffs between throughput, delay statistics and fairness. A scheduler known as a proportional fair (PF) scheduler has been developed to provide improved throughput and fairness over the downlink of the High Data Rate (HDR) standard. It has been shown that the PF scheduler maximizes the sum of the logarithms of the MS's average throughput. In another scheduler, a fraction of timeslots allocated to each user can be arbitrarily chosen based on desired objectives (such as meeting desired Quality of Service (QoS) requirements), and the scheduler then maximizes the average system throughput under the time allocation constraints. A further scheduling algorithm, known as a Modified Largest Weighted Delay First algorithm, supports n MSs with desired Quality of Service (QoS) based on the expression: Pr{Wi>τi}≦δi, i=1, . . . , n. In the above expression Wi is the packet delay for MS i, τi represents a delay threshold, and δi represents the maximum probability of exceeding the delay threshold, respectively.
Each of the above exemplify schedulers or scheduling algorithms employ OS techniques. However, when the coherence time of the propagation channels is long in comparison with an acceptable packet delay, or when the fading is weak (e.g., Ricean fading with high K factor), OS is not useful.
Accordingly, to address the slow channel or weak fading scenarios, a scheduling technique know as Opportunistic Beamforming (OBF) has been proposed. OBF is a “natural” enhancement to OS that, in general, amounts to replacing a base station having a single antenna with multiple antennas and implementing an algorithm that generates a different radiation pattern, henceforth referred to as a ‘beam’, every timeslot. The sequence of beams may be completely random and a-periodic, or could be chosen pseudo-randomly, from a fixed pre-designed collection of N beams. OBF techniques may be distinguished from other proposed downlink beamforming systems by the simplicity of the techniques. For example, and similar to OS, OBF requires a small amount of uplink feedback, while the beam generating algorithm runs independently, i.e., does not receive any information from the rest of the system.
Communication systems employing OBF techniques have been considered in two different environments, an uncorrelated environment and a correlated environment. In an uncorrelated environment, propagation channels between a MS and any of the BS's antennas are all uncorrelated. A realistic system comes close to this case when the antennas of each BS are placed sufficiently apart (e.g., 10 feet at 2 GHz) and there is some scattering in the environment. In a correlated environment, the propagation channels between any MS and all the antennas of a single BS are highly correlated, but channels to different BSs are uncorrelated. A realistic system comes close to this case when the BS's antennas are placed close to each other, (e.g., one half wavelength apart), and the angle spread at the BS is low. As it has been shown that, in general, a correlated system outperforms an uncorrelated system, the correlated system is therefore of focus herein.
In currently developed OBF techniques, high throughput and good delay properties are conflicting objectives. Studies on OBF techniques have primarily focused on the scheduling algorithm alone, while the beam generating algorithm for a correlated system was taken to be a simple periodic scanning of the desired cell/sector. The beam generating algorithm was running independently from the scheduler, and no attempt has been made to use system state information to modify the beam sequence in any way. A joint study on beamforming and scheduling has not heretofore believed to have been conducted